The objective of this cohort study is to assess the incidence density ratio of lung cancer over a 25 year period in the defined population of the Tecumseh Community Health Study by comparing participants who were cigarette smokers with symptoms and/or impaired pulmonary function indicative of chronic obstructive pulmonary disease (COPD), with participants of similar demographic characteristics and cigarette smoking exposure history, but without evidence of COPD. The association of lung cancer incidence with symptoms of COPD and level of impaired ventilatory functions (FEV1.0, FEV1.0/FVC, MMEF, Vmax50%) expressed as percent of predicted will be explored. The study population will be derived from a chort of 4,026 men and women who were 25 years of age and older during 1962-65, at which time pulmonary functions were measured in conjunction with the administration of a comprehensive lifestyle risk factor questionnaire and physical examination. The Cox proportional hazards method will be used to estimate incidence density of lung cancer among various strata of COPD and non-COPD adjusting for age, sex, socioeconomic status, cigarette smoking, alcohol consumption, dietary beta carotene, vitamin A and fat consumption and occupational exposures. Independent association of COPD with lung cancer after controlling for potential confounders will indicate susceptibility in COPD patients of bronchopulmonary carcinogenesis. We may only speculate that the mechanism may be due to an impaired ability to clear or filter biologically active tobacco smoke constituents deposited in the respiratory tract and/or to the interaction of tobacco smoke carcinogens with antecedent proteolytic injury to bronchial epithelium.